Optoelectronic devices such as light-emitting diodes (LEDs) emit light in response to an excitation signal. A typical LED is a heterostructure that has been deposited on a host substrate via a growth technique such as liquid phase, hydride phase, molecular beam, and metal-organic phase epitaxy. The heterostructure includes n and p-type semiconductor layers that sandwich active light producing layers. Electrical contacts are attached to the n and p-type semiconductor layers. When a forward voltage is applied across the contacts, electrons and holes are injected from the n and p-type semiconductor layers into the active region. Light is produced when the electrons and holes radiatively recombine in the active layer(s).
Wall-plug efficiency is the amount of light power produced compared to the electrical power applied. High wall-plug efficiency can be achieved by maximizing the total efficiency of the device. The total efficiency of the device is a product of the various efficiencies of the device including the injection, internal quantum, and light extraction efficiencies. The first two parameters depend on the material quality of the device (epitaxial growth and electronic band structure) while the light extraction efficiency depends on the geometry and all the light absorption present in the device. The light extraction efficiency is the ratio of the amount of light leaving the LED compared to the amount of light generated inside the LED. One way to increase the light extraction efficiency is to reduce the absorption and redirect light into useful (higher extraction) directions. Therefore, absorbing paths in the device should be avoided and light should be scattered into the proper escape cones of the device. The angle of the escape cones depend on the refraction indices of the light-producing semiconductor and the exiting medium, (e.g. for GaN into air the angle of the escape cone is 25°). The electrical contacts are one example of light absorption in a typical LED. Therefore, it is preferable to reduce the absorption and for some devices also to increase the reflectance in these electrical contacts. This must be done without compromising the contact resistance. Resistance in the electrical contacts leads to wasted energy (electricity) thus lowering the wall-plug efficiency.
Highly reflective ohmic contacts are desirable in LEDs. There are many prior art approaches to creating these types of contacts. The simplest way is to use a thick sheet of the ohmic contact metal. This thick sheet acts as a contact and reflector. A good reflector is one that absorbs less than 25% from an incident hemispherical isotropic light source. Therefore, isotropic light will lose less than 25% of its intensity after reflecting off of this medium (e.g. a maximum reflection of >75%). For the entire visible spectrum (400 nm–750 nm), this leaves two metals that fit the requirement: Al and Ag. Other metals that work in only parts of the visible spectrum are Au, Rh, Cu, and Pd. Although a single thick sheet is preferred, these metals do not always make good ohmic contacts to the selected material system. There are additional reliability issues with the use of Ag because of electromigration, and Cu because it may diffuse into the light-producing active region thereby creating deep levels in some semiconductor materials hindering light output.
One prior art approach, disclosed by Chai, et al. in U.S. Pat. No. 4,355,196, is to pattern the ohmic contact metal, and overlay the ohmic patterned metal with a reflective metal. Although Chai, et al. teaches a reflective contact with a solar cell device, the idea can be extended to all other optoelectronic devices including LEDs. This patterned contact is not advantageous when used on a device with semiconductor layers that do not spread current efficiently laterally (i.e. a low conductivity semiconductor such as p-GaN with a resistivity, ρ, greater than 0.5 Ω-cm). The low conductivity semiconductor cannot spread current efficiently from the patterned contact; therefore electrical carriers will not be injected uniformly into the active light-producing region of the device. Non-uniform injection reduces the wall-plug efficiency of the device. Also, patterning of the contacts adds additional complicated processing steps. Any non-uniformity of the pattern will be manifest in non-uniform current injection and light generation. For low conductivity semiconductor devices, the ohmic contact needs to be a uniform sheet. This type of approach is described by Aegenheister, et al. in EP0051172, although not for reasons of uniformly injecting in low conductivity semiconductor devices. It teaches using an Au/Ge (ratio 99:1) ohmic layer that is 200 Å thick. Although this ohmic contact layer is thin for a long wavelength emitting device, this contact is too thick for a device emitting in the visible spectrum (i.e. at 505 nm absorption ˜29%). Also, the overlaying reflective metal is Ag. Ag is known to electro-migrate (when used as a p-contact) in devices that operate with high electric fields in humid environment (accelerated) life tests, thus shorting out the device and rendering it useless. Therefore, electro-migrating electrical contacts are not useful in commercial LEDs. A multi-layer highly reflective ohmic contact is also described in P. M. Mensz et. al., Elec. Lett., 33, 2066–2068 (1997) where the contact is Ni/Al or Ni/Ag to p-GaN for a GaN based LED. This approach is also problematic because its operating forward voltage (Vf) is 5 V at 20 mA (for a 300 μm×300 μm contact area). This voltage is 1.5–2.0 V too high for an GaN LED of that size, indicating that the contact is not ohmic and that specific contact resistance is too high. The additional contact resistance decreases the wall-plug efficiency of the LED device.